The present invention relates to refrigeration systems, and more particularly to an absorption refrigeration system powered by solar energy.
The heating of buildings through the use of solar energy involves the mounting of external solar collectors which absorb the sun's heat into a heat transfer fluid, usually water. Heated water is then circulated or stored for later use in heating the building. Since, in warmer climates, the energy used in cooling a building may exceed that used in heating the building, efforts have also been made to utilize solar energy to cool and air condition buildings.
One approach has been the use of a solar energy powered absorption refrigeration system. Absorption refrigeration systems in general are well known in the art. They commonly employ steam or a gas flame to vaporize a refrigerant from a solution of a refrigerant and an absorbent. In a solar energy application, solar heated hot water at about 200.degree. F. (93.degree. C.) has been used to heat the solution in the generator. Water has been typically used as the refrigerant; lithium bromide has been typically used as the absorber because it has a very strong affinity for water vapor. The solution in the generator boils violently, carrying water vapor and concentrated lithium bromide up to a separator, where the two are separated by baffles. The vapor passes to a condenser, where it is condensed, at 0.25 atm (50-60 mm Hg) and then passes through an expansion device to the low pressure side of the system where the water vaporizes at 6 to 9 mm Hg (40.degree. F., 4.degree. C.) causing a cooling effect on circulating water or air passed across the evaporator tubes. From the evaporator, the expanded water vapor passes to an absorber. The absorber contains concentrated solution which flows by gravity from the separator of the generator and is trickled over a cooling coil. The vapor from the evaporator is absorbed into the solution on the cooling coil and the reconstituted solution is passed against a heat exchanger containing hot concentrated solution passing from the generator to the absorber. The reconstituted solution is returned to the generator in an absorber-generator loop to complete the cycle. Outside water is used in the cooling coil of the absorber and the condenser.
The solar powered absorption refrigeration system uses water at 200.degree.-210.degree. F. to boil the solution in the generator. This water is heated by solar panels. With improvements in the generator heat exchange surface, the addition of a mechanical solution pump and the use of separate cooling streams for the condenser and absorber, a three ton unit may be operated at full capacity at 195.degree. F. water temperature.
Another approach to solar energy powered cooling has been the co-called Rankine-cycle solar air conditioner. A Rankine cycle refers to the successive compression, heating, expansion, and condensation of a working fluid in a heat engine, such as a steam engine. The cycle may either be closed, where the working fluid is recycled, or open, where the expanded and condensing steam is simply vented to the air. In the Rankine cycle solar air conditioner, solar heated hot water at 215.degree. F. (102.degree. C.) from solar panels enters a multiple stage boiler where a working fluid of a liquid refrigerant, R-113 (trichlorotrifluoroethane) is vaporized and used to turn a small turbine. The expanded refrigerant is cooled, condensed and pump returned to the boiler. The turbine is used to drive a compressor in a conventional vapor compression refrigeration system.
In a conventional vapor compression refrigeration system, a refrigerant, which is a compressible, condensible gas, is compressed in a compressor, passed to a condenser where the gas is condensed to a liquid, and passed through an expansion device to a low pressure evaporator, where evaporation and cooling take place. The compressor in the Rankine cycle solar vapor compression system is driven either by the turbine or by a back-up electric motor.
The need for back-up systems for night-time or marginal weather conditions is presently a principal problem in the development of solar energy cooling systems. In the case of solar energy absorption refrigeration systems, a complete vapor compression system has been used as a back-up. The use of back-up systems in prior art solar cooling systems has been unsatisfactory because of the necessity of high temperatures to power the system. Back-up systems must therefore be used under marginal conditions, when solar energy is available but the solar heated water is at a temperature below that needed to power the system. In a solar energy absorption refrigeration system there is a temperature, typically around 160.degree. F. (71.degree. C.), below which no refrigeration is possible due to lack of vaporization in the generator. Furthermore, as the generator temperature falls from the optimum, the coefficient of performance (C.O.P.) or efficiency of the system is also decreased.
F. A. Costello, in U.S. Pat. No. 4,031,712, describes a combined absorption and vapor compression system for solar energy applications. In this system, a compressor is added to a standard generator-condenser--evaporator--absorber loop either between the evaporator and absorber, or between the generator and condenser, or preferably, at both points. A refrigerant line is constructed between the compressors to form a compressor--condenser--evaporator loop.
Due to the low C.O.P.'s of prior art solar energy cooling systems, a large amount of heat must be put into the system to obtain sufficient cooling output. This necessitates an additional expense in the use of high performance collectors such as concentrators, parabolic sun trackers, evacuated tube collectors, etc. If low performance flat plate collectors are used, their area must be increased, at a corresponding increase in cost.
Accordingly, it is an object of the present invention to provide a thermally efficient refrigeration system.
It is another object of the present invention to provide a solar energy powered refrigeration system which needs no separate back-up system.
It is a further object of the present invention to provide a solar energy powered refrigeration system which can utilize low grade solar energy.
It is yet another object of the present invention to provide a solar energy powered refrigeration system which can utilize low performance solar collectors.
It is yet another object of this invention to accomplish the above mentioned objectives using a standard oiled compressor to assist an absorption refrigeration system.